arc_summary3: Handle overflowing value width

Some tunables shown by arc_summary3 have string values that may exceed
the normal line length, leaving a negative offset between the name and
value fields.  The negative space is of course not valid and Python
rightly barfs up an exception traceback.

Handle an overflowing value field width by ignoring the line length
and separating the name from the value by a single space instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11270

cmd/arc_summary/arc_summary3

@@ -388,8 +388,12 @@ def format_raw_line(name, value):
     if ARGS.alt:
         result = '{0}{1}={2}'.format(INDENT, name, value)
     else:
-        spc = LINE_LENGTH-(len(INDENT)+len(value))
-        result = '{0}{1:<{spc}}{2}'.format(INDENT, name, value, spc=spc)
+        # Right-align the value within the line length if it fits,
+        # otherwise just separate it from the name by a single space.
+        fit = LINE_LENGTH - len(INDENT) - len(name)
+        overflow = len(value) + 1
+        w = max(fit, overflow)
+        result = '{0}{1}{2:>{w}}'.format(INDENT, name, value, w=w)
 
     return result